The objects you can observe in nature come in different sizes. Some of them can be seen with the naked eye. However, when we use a school microscope or a magnifying glass, we can smaller objects or even individual cells.

The largest human cell, the ovum (egg cell), can be distinguished with the naked eye.

However, when studying human biology, even smaller things, such as parts of a cell, need to be observed. For this reason, medical research makes use of precision microscopes. Things like pathogens, bacteria and viruses cannot be seen without microscopes.

Genetic factors must be studied by methods other than microscopy. The chart below illustrates sizes of various organisms in a scale from one kilometer to one tenth of a nanometer.